1. Field of the Invention
The present invention relates to processes that remove sulfur and nitrogen impurities from carbon-based solid fuels such as calcined petroleum coke or coal. More particularly, this invention relates to the desulfurization of calcined delayed petroleum coke by controlled contact with Carbon Monoxide (CO) gas at elevated temperatures to produce a high-grade coke suitable for metallurgical and chemical applications. Raw coke produced from Delayed coke processes typically contains high levels of sulfur and nitrogen impurities. These impurities are both chemically and physically embedded within the carbon, which renders them unsuitable for many important commercial uses. However, the relatively low cost and abundance of raw petroleum coke as a byproduct of crude oil refining has created much demand for processes that can economically remove these impurities. The demand for newer, more efficient coke desulfurizing processes is becoming even more imperative as crude oil sulfur levels have been increasing over the years. Additionally, processes for desulfurizing coal have also become imperative as more stringent pollution regulations are placed on coal-fired power plants.
High-value coke, such as that used for making anodes for aluminum production and electrodes for ferrous metallurgy are predominantly made from calcined Delayed coke. Delayed coke is obtained by heating liquid crude oil residuum to around 480° C.-500° C. and holding the material in a large insulated vessel for several hours. During this holding time, the tarry hydrocarbon molecules crack into lighter gaseous fractions. As the remaining mixture cools, it solidifies into a material called Delayed coke, which contains mostly elemental carbon, some very large hydrocarbon molecules (volatile matter), minerals, and various other inorganic impurities. The crystalline structure of the carbon is dependent on the type of coking process used to create it. The most abundant type, “sponge coke,” is a porous, crystalline material that, after calcining to remove the volatile matter and to refine the structure, is a suitable ingredient for typical carbon products, such as carbon and stainless steels. A second common form is called “needle coke.” Needle coke is even more crystalline than sponge coke and provides even greater hardness and strength, which makes it more suitable for products such as electrodes for metallurgy manufacturing.
However, the various sulfur and nitrogen impurities within raw Delayed coke would render the material useless for such high-value applications. Their presence causes structural deficiencies and other undesirable qualities in the end carbon product. Sulfur impurities are typically the higher concentration of the two and are a function of the sulfur content of the crude oil from which the coke is derived. The process of calcining generally does not heat the coke to high enough temperatures to remove sulfur. This invention provides a process that utilizes high temperature and chemical reactivity to desulfurize delayed coke to produce a high-value coke suitable for use in carbon products.
A second type of coke making process, referred to a Fluid coking, generally produces coke of unacceptable quality for electrode and steel production without special treatment. Fluid coking converts heavy crude tars into a coke that has a relatively amorphous crystalline structure. This amorphous structure lacks the required hardness, strength, bonding and handling characteristics required in the manufacture of electrodes and other high-value carbon products. Accordingly, fluid coke is considered a “low-value” carbon fuel and is typically burned as solid fuels in boilers similar to coal. Still, some fluid coke is used in production of carbon products usually by grinding and blending with Delayed coke to minimize the adverse impacts on end product quality. This invention provides a process that substantially desulfurizes fluid coke to produce a more useful carbon product.
2. Description of the Prior Art
The primary process parameters for most all petroleum coke desulfurization processes are: 1) temperature, 2) residence time, 3) particle size of the material being treated, 4) contact with a chemical reagent, and 5) pressure. Most desulfurizing methods disclosed in the prior art involve elevated temperatures (<1,300° C.) at set residence times (1-24 hours) or involve contacting the coke with chemically reactive agents (hydrogen gas, liquid acids, sulfur gases, etc.). Desulfurization methods that contact coke with an inert gas are generally categorized with the temperature/time processes and have limited sulfur removal efficiency.
Many prior publications and patents describe efforts to use a hot, inert flue gas to desulfurize coke. The Oil and Gas Journal, Jan. 22, 1979, pg. 64-68, describes a thermal process whereby the coke is heated to a set temperature over a three to nine hour period. One drawback of thermal-time processes is that high sulfur removal requires high temperatures, which often results in loss of coke yield and product density.
U.S. Pat. No. 4,160,814 discloses a thermal process with data showing extensive desulfurization with nitrogen as the inert medium but there is no suggestion that other gases may be used.
U.S. Pat. No. 3,009,781 describes a two-stage process for treatment of fluid coke in which the first stage involves electrothermic production of carbon disulfide from the coke and a second stage involved passing a stream of gas through the bed. This patent states that gases such as nitrogen, carbon monoxide, hydrogen, mixtures of carbon monoxide, hydrogen and nitrogen, and hydrogen sulfide are especially effective desulfurizing agents at 1,500° C. This patent further states that sulfur removal by these gases (excluding nitrogen) is enhanced if the coke is pre-treated with an oxidizing gas and an alkali metal. One problem with this patent is that the inclusion of nitrogen, an inert gas, in a list with other gases known to chemically react with sulfur suggest the inventors could not distinguish thermal desulfurization effects from chemical desulfurization effects.
U.S. Pat. No. 4,011,303 discloses a process whereby a gas containing sulfur is used to chemically remove the sulfur from the coke, without reference to pre-heating. The reagent gas is elemental sulfur vapor diluted with nitrogen whereby the elemental sulfur combines with carbon-sulfur groups in the coke. However, it also reacts with carbon in the coke, contributing to a loss of carbon yield. The sulfur removal efficiency of this process is generally low.
In U.S. Pat. No. 4,406,872, Delayed coke is desulfurized by contact with an active sulfur-bearing gas at a temperature high enough to produce a reaction between the sulfur in the coke and the active sulfur in the gas, and then holding the coke in contact with the gas at said high temperature for approximately one hour. Two alternatives are presented, one involving a modified second step having contact with an inert gas and a second involving lowering the temperature during the second step to maintain the reaction between the sulfur-bearing gas and the coke.
In U.S. Pat. No. 4,511,362, carbonaceous materials are desulfurized using an up-flow fluidized bed reactor where chlorine gas at a temperature up to 300° C. is injected until at least 1 percent by weight of chlorine is added to the material. Next, the chlorine gas is exchanged with an inert gas at a temperature of at least 300° C. to dechlorinate the material below 1 percent by weight.
In U.S. Pat. No. 3,130,133, substantial sulfur removal is accomplished by subjecting the treating material sized to 200 mesh to a hydrogen gas stream flowing through and that reacts with the sulfur to form gaseous Hydrogen Sulfide. This patent establishes that significant sulfur removal can be achieved by reaction with hydrogen gas at a relatively low temperatures (94% impurity removal at 750° C.). However, this patent also states that such high removal rates at such low temperatures require that the coke be pre-ground and screened to 200 mesh presumably to increase the coke surface area where the hydrogen-sulfur reactions take place.